Thermoplastic resins such as poly(vinyl chloride) are commonly produced in particulate form, e.g. as pellets or powders. When the particulate resin is employed to manufacture an article, as by molding, extrusion, etc., the particles must first be melted and formed into a homogenous mass. In the past, this process of fusion has been aided through incorporation into the resin of primary plasticizers, which act as solvents for the resin. The plasticizers also improve the flow and shear characteristics of the resins during processing into the final article. That is, the molten resin is more readily formed into the desired shape due to its reduced viscosity. Although plasticizers have been widely employed to assist in molding thermoplastic resins, they are not entirely satisfactory in all cases. In some cases, it is desired that the fabricated article have physical properties, especially rigidity and stiffness, comparable to those of the thermoplastic resin itself. It has been found, however, that when primary plasticizers are employed in amounts sufficient to improve the processability of thermoplastic resins, they materially reduce the rigidity of the resin. It is desirable, therefore, that there be a processing aid which can be blended with a thermoplastic resin such as poly(vinyl chloride) in amounts sufficient to improve the processability of the particulate resin during fabrication without also materially reducing the rigidity of the fabricated resin.